Method of modifying the acoustics of a room



Feb. 8, 1938. oux ET AL 2,107,804

METHOD OF MODIFYING THE ACOUSTICS OF A ROOM Filed Dec. 20, 1935 Patented Feb. 8, 1938 PATENT OFFICE METHOD OF MODIFYING THE ACOUSTICS.

OF A ROOM Bernard Roux, Robert Gamzon, and Marlo Sollima, Paris, France Application December 20, 1935, Serial No. 55,474 Iii-France December 22, 1934 Claims.

This invention relates to a new or improved method of and means for improving or correcting the acoustics of a. room, hall, studio or other enclosed space or auditorium.

Architects have experienced great difficulty in producing rooms having good acoustic qualities, which latter must include not only good intelligible hearing of speech but also an agreeable colour or tone in the hearing of musical performances. This difficulty, which occurs with rooms or halls intended for direct audition, is greater in the case of sound studios wherein microphones pick up sound to be transmitted or recorded. With such studios, the fear of sound confusion resulting from echo has conduced to the employment of absorbent materials. This imparts a dead and dull tone which is very disagreeable from the point of view of musical aesthetics.

Recent studies have shown that, once the echoes are avoided, the artistic sound quality of a hall or studio depends solely upon reverberation characteristicsfor the different musical frequencies. The reverberation period of time (this being the time during which the intensity of a simple sound, having its source of emission sharply arrested, falls to one millionth of its value) is not the same in one and the same room for all frequencies. It has been shown that a hall, having acoustics suitable for speech and music, should have a greater reverberation period for deep and sharp or shrill sounds than for medium sounds. For the best effect, the reverberation period should vary as a function of the 3 frequency following an optimum curve which, generally speaking, should be approached as closely as possible.

When troublesome echoes have been avoided in. a hall or studio by suitable architectural or acoustic arrangements, it is often found that the reverberation curve has no relation whatever to,

the emitted sound is transmitted to a reverbera tion chamber having a large reverberation period but of small dimensions for avoiding echoes. A microphone placed in this chamber receives the sound reverberated. If it is a matter of correcting or modifying the acoustics of a hall in which an audience has direct hearing, the reverberated sound is returned by a system of amplifiers and loud speakers and is superimposed upon the sound emitted directly in the hall. In the case of a sound studio, however, the reverberated sound may be mixed electrically with the sound normally picked up in such studio.

The invention further provides for the introduction of one or more electrical filter devices, these devices having the function of afiecting the sound intensity corresponding to a given frequency or frequencies. In practice, the more general arrangement would include a filter or filters affecting the intensities of the sounds of dififerent zones or bands of musical frequencies according to a given coeflicient or coefllcients of weakening regulatable at the will of the operator. The filters aforesaid may be introduced in A the circuit from the hall to the reverberation chamber, or in the circuit from the reverberation chamber to the hall, or in the circuit from the reverberation chamber to the electrical mixer.

The total sound (direct sound plus reverberated sound)--or the equivalent modulations of electric current-then has, for each given frequency, a period of apparent reverberation which is greater as the aforesaid coefiicient of weakening is less for that frequency. The maximum reverberation period, for a frequency or group of frequencies, is that of the reverberation chamber for that frequency or group of frequencies.

In practice, and for any given auditorium, it is possible to obtain a desired curve, as follows:

1) Take the real curve representing the reverberation period of the auditorium as a function of the frequency.

(2) Compare that curve with the curve that it is desired to obtain, for example, the optimum curve. (The problem can be resolved by this method only if the real curve does not present, for any frequency, a reverberation period greater than the corresponding period for the same frequency on the desired. curve. This condition can always be realized by. sufiiciently deadening in its entirety the auditorium or studio concerned.)

(3) From the comparison is deduced the reverberation period that, for each given frequency or zone of frequencies, should be added to the natural reverberation period of the auditorium or studio for that frequency or zone of frequenci'es.

As already indicated, the required addition is obtained by means of a reverberation chamber and one or more filters, the latter being adapted for giving to the appropriate frequency or group of frequencies the desired lengthening of the reverberation period.

The result of the foregoing intuitive reasoning can be made mathematically precise, as follows:-

Consider a reverberation curve giving sound intensity as a function of time for a given frequency, the origin of time being the arrest of sound emission from its source. The scale of intensity is logarithmic and the scale of time is linear. The curve of reverberation for a given frequency in the reverberation chamber is represented by a straight line A--B see Figure 4 of the accompanying drawing. The period of reverberation is 0-3, the point 0 being decibels beneath A. Let it be assumed, for simplicity, that f the auditorium, for the frequency in question, has no reverberation, then, if the filters and amplifiers are regulated for giving no weakening to the transmission of the sound reverberated, the total period of reverberation will be OB.

If, by regulation of the filters and amplifiers a weakening of, say, 10 decibels is effected in the sound reverberated at the frequency in question, the curve will be displaced 10 decibels parallel with itself and the total curve will be A-A --B the apparent period of reverberation being 0-43 which is less than 0-3.

The regulation of the weakening of the intensity for each frequency (a weakening that can be obtained and regulated by known means) determines therefore the apparent reverberation period for each frequency. In Figure 4, the reverberation period varies as the logarithm of the initial intensity of the sound reverberated.

If a weakening of 10 decibels is introduced in the circuit of the sound to be reverberated, the total period of reverberation is reduced onl'yone sixth of its previous value. On the other hand, and with the same conditions, the sound passing through the reverberation system, being 10 decibels weaker than the direct sound, can be considered as negligible relatively to the latter. This shows that the regulation of the filters of a. system in accordance with the invention cannot, in practice, appreciably modify the curve of transmission of total resultant intensities from the initial source of sound to the sound recording or receiving apparatus, or to the ear of the audience. Its eifect is manifested only upon the duration of reverberation.

The annexed drawing diagrammatically represents different examples of means for carrying out the invention.

Figures 1 and 2 are diagrams showing simple circuits leading to the reverberation chamber.

Figure 3 is a diagram'showing a triple branch circuit leading to the reverberation chamber, and

Figure 4 is a diagram showing curves already referred to.

In Figures 1 to 3, S indicates an auditorium, hall, or sound studio. C is the reverberation chamber. In the auditorium, a microphone 1n produces electrical modulations corresponding to the sounds emitted. D is a device for deviating a part of the modulations on to a branch circuit ending in a loud speaker H in the reverberation chamber C. Electrical amplifiers are indicated at -Pl, P2, P3, PI and P5, and the coefficient of amplification by these instruments may be regulatable or not.

In the chamber C a second microphone 1: picks up the sound reverberated in that chamber and introduces it to a mixer M where it is mixed with the sound transmitted directly from the microphone m.

In Figure 1 an electric filter F is placed between the microphone n and the mixer M for regulating the relation of the intensities for the different frequencies.

4 In Figure 2 a filter Fl is placed between the device D and the loud speaker H.

In Figure 3, the circuit from the device D comprises three branches each including a regulatable amplifier, as indicated at PI, P2, and P3. Each branch also includes a filter indicated at Fb, Fm and Fit, and ends at a loud speaker in the chamber C, these loud speakers being indicated at HI, H2, and H3.

The three filters Fb, Fm, Fh, are regulated so that each permits passage of an appropriate part of a sound. Thus, for example, the filter Fb may be a low-pass filter permitting low tones to pass; the filter Fm may be a band-pass filter for passing medium tones,' and the filter Fh may be a high-pass filter for the elevated tones.

The sound thus filtered is transmitted to the loud speakers HI, H2, H3. The acoustic mixture of sounds emitted by the loud speakers is reverberated in the chamber C and is then picked up a by the microphone n and transmitted through the amplifier P4 to the mixer M. In this example, regulation of the reverberation curve can be accomplished by means of potentiometers, for instance, operating upon the amplification of the amplifiers Pl, P2, P3.

The several examples relate to the case where the reverberated part of the transmission is mixed electrically with the part directly transmitted. Thus, they are concerned more particularly, with the taking of records on discs, films and so on, or with the emissions of wireless telegraphy.

It is to be understood that analogous arrangements could be utilized for the transmission to an auditorium of the part reverberated only. In this case, the circuit of the microphone n would include one or several loud speakers placed in the auditorium at one or more suitable points.

Further, it is to be understood that the invention is not limited to the particular examples hereinbefore described and illustrated by the drawing. In particular, it is not necessary that both the transmission of sound to the reverberation chamber and the transmission of reverberated sound from that chamber, should be effected by electrical means. One of the transmissions could be eflected by electrical means and the other by acoustic means. 01! course, the means for varying the intensity of the sound reverberated as a function of the frequencies will be in that part of the transmission which is electrical.

Having now' particularly described and ascertained the nature of our said invention. and in what manner the same is to be performed, we declare that what we claim is:-

1. In an electro-acoustical apparatus for transmitting sounds, a sound transmitter, an acoustical echo device for artificially lengthening the reverberation period of the transmitted sounds, means for conveying to the echo device a part of the energy coming from the transmitter, means for picking up the corresponding energy from the echo device and leading it to a receiver, one of the last named means being electrical, and electrical means combined with said electrical conveying means for affecting the intensities of the different musical frequencies conveyed, according to given coefllcients of weakening regulatable at the will of the operator.

2. In electro-acoustical apparatus for the transmission of sounds, a transmitter for converting the sounds emitted into electrical currents, an acoustical echo room, means for carrying a part of said electrical currents to said echo room, means for converting the electrical currents into acoustical vibrations in said echo room, means for picking the reverberated vibrations from the echo room, means for again converting these vibrations into electrical currents, means for modifying at will the intensity of each of the various frequencies or groups of frequencies of the electrical currents leading to or coming from the echo room, means for receiving the currents coming directly from the transmitter as well as the currents coming from the echo room.

3. In electro-acoustical apparatus for the transmission of sounds, a transmitter for converting emitted sounds into electrical currents, means for directly leading a part of the electrical energy so generated to a receiving apparatus, means for carrying another part of said electrical current into a plurality of branches of current, a potentiometer and a selective filter on each branch, an echo room having loud speakers located therein, said branches leading to a loud speaker in said echo room, one or more microphones in the echo room and one or more electrical circuits leading from said microphones to the first named receiving apparatus.

4. In an electro-acoustical apparatus for the transmission of sounds, a transmitter for converting emitted sounds into electrical currents, an acoustical echo room, means for leading a part of said currents to said acoustical echo room, means for converting the electrical currents into acoustical vibrations in said echo room, means for picking the reverberated vibrations from the echo room, means for again converting said vibrations into electrical currents, means for dividingsaid currents into a plurality of branches, a potentiometer and a selective filter on each branch, and means for receiving the currents coming directly from the transmitter as well as the currents carried by the several branches.

5. In an electro-acoustical apparatus for the transmission of sounds, a non-damped auditorium, a transmitter located in said auditorium for converting a part of sounds emitted in said auditorium into electrical currents, an acoustical echo room, means for leading said currents to said acoustical echo room, means for picking the sounds produced by these currents in said room after reverberation, means for again converting these sounds into electrical currents, means for converting said currents into sounds, means for modifying at will the intensity of each of the various frequencies or groups of frequencies of the electrical currents leading to or coming from the echo room and means for reinjecting into the auditorium the sounds produced by the currents coming from the echo room.

. BERNARD ROUX.

ROBERT GAMZON. MARIO SOLLIMA. 

